Diesel particulate filter control

ABSTRACT

Methods and systems are provided for managing particulate emissions in an engine including a particulate filter and a CO 2  sensor downstream of the filter. A CO 2  sensor may be used to infer the presence of particulate matter in the exhaust, downstream of the filter. By sensing particulate matter in the post-filter exhaust, filter degradation may be identified.

FIELD

The present description relates to particulate matter purifying systems for internal combustion engines, such as diesel engines and gasoline engines.

BACKGROUND AND SUMMARY

Emission control devices, such as diesel particulate filters (DPF), may reduce the amount of particulate matter emissions (such as, soot) from a diesel engine by trapping the particles. Such devices may be regenerated during engine operation to decrease the amount of trapped particulate matter (for example, by burning) and maintain the collection capacity of the device. To meet stringent federal government emissions standards, regeneration operations and DPF functionality may be tightly controlled and regularly assessed.

One example approach for controlling diesel particulate filters is illustrated by Stewart et al. in U.S. Pat. No. 7,155,334. Therein, an engine controller initiates filter regeneration based on inputs received from sensors, such as particulate matter sensors and/or carbon dioxide sensors, positioned upstream and downstream of the filter. Specifically, the controller determines whether to regenerate the filter based on the composition of exhaust gas before and after passage through the filter.

However, the inventor herein has recognized issues with such an approach. As one example, the use of resistive sensing-based particulate matter (PM) sensors reduces the sensitivity of the emission control system. As such, commonly used PM sensors may be configured to detect the presence of PMs electrically, based on a change in resistance or capacitance across an electrical circuit. Such sensors may have a “dead-band” during which PMs may have to accumulate before the sensor is able to respond. This additional time required to detect PMs may delay the determination and initiation of filter regeneration. As such, the delay in filter regeneration may accelerate filter degradation. The additional time may also reduce the electrical sensor's ability to identify DPF degradation. Overall, this may lead to degraded exhaust emission levels.

As another example, the use of input from CO₂ sensors that sense exhaust CO₂ levels in determining filter regeneration may reduce the system's ability to accurately estimate the soot load on the filter due to an indirect correlation between filter soot levels and exhaust CO₂ levels. Since the exhaust CO₂ level is more representative of combustion conditions, a soot load may be inferred but not accurately determined.

Thus, in one example, some of the above issues may be addressed by a method of operating an engine exhaust system including a particulate filter comprising adjusting engine operation based on a CO₂ signature of oxidized, post-filter exhaust particulate matters (PMs). The CO₂ signature may include a CO₂ level of oxidized PMs estimated by a CO₂ sensor positioned downstream of the filter.

In one example, a diesel engine exhaust system may be configured with a filter substrate and a CO₂ sensor positioned downstream of a DPF. During selected engine running conditions, an engine controller may heat the substrate and oxidize post-filter exhaust particulate matters (that is, exhaust soot) on the heated substrate using oxygen present in the exhaust gas. The CO₂ generated from the oxidation of the soot may be estimated by the downstream CO₂ sensor to determine a CO₂ signature of the oxidized, post-filter exhaust particulate matters (PMs). The CO₂ signature may at least include a CO₂ level of the oxidized PMs. Since the generated CO₂ is largely dependent on the quantity of exhaust soot oxidized on the heated substrate, a direct correlation may be made between the estimated exhaust CO₂ level and an exhaust soot level. In other words, the CO₂ sensor may be used as a PM sensor. The controller may then adjust engine operation and perform filter diagnostics based on the CO₂ signature. During other engine running conditions, the CO₂ sensor may be used to sense an exhaust CO₂ level unrelated to post-filter exhaust PMs.

For example, during a first engine running condition, when the filter is storing and the substrate is not oxidizing (e.g., a substrate heater is turned off), the engine controller may adjust engine operations (such as, EGR operations) based on an exhaust CO₂ level estimated by the CO₂ sensor. During a second engine running condition, different from the first engine running condition, when the filter is storing and the substrate is oxidizing (e.g., a substrate heater is turned on), the engine controller may adjust engine operations (such as, initiation of filter regeneration) and perform filter diagnostics (such as, determine that a filter is leaking) based on the estimated CO₂ level of oxidized, post-filter exhaust PMs. As such, during such conditions, low to substantially no exhaust PMs may be expected in the post-filter exhaust. Herein, by comparing the output of the CO₂ sensor to CO₂ levels expected based on the engine's operating conditions, exhaust PMs may be identified in the post-filter exhaust and may be used to infer filter degradation. For example, the CO₂ sensor may sense post-filter exhaust CO₂ levels in real-time to provide a real-time indication of the presence of PMs in the exhaust. In response to the estimated CO₂ level (that is, sensor output) being greater than the expected CO₂ level, an engine controller may indicate filter degradation due to the presence of PMs in the exhaust. In the event of filter degradation, the controller may further adjust engine operating conditions, and regeneration conditions, in response to the indication of degradation. In comparison, if the estimated level is not greater than the expected level, but is greater than a threshold, the controller may infer that the filter is not degraded, but that sufficient soot has accumulated on the upstream filter, and that a filter regeneration operation should be initiated. Accordingly, filter regeneration may be initiated to restore the filter's collection capacity.

It will be appreciated that while the depicted example illustrates application of the CO₂ sensor in a diesel engine exhaust system, this is not meant to be limiting, and the same CO₂ sensor may be similarly applied in alternate engine exhaust systems, such as to diagnose a gasoline particulate filter in a gasoline engine exhaust system.

In this way, the presence of soot in engine exhaust may be detected by oxidizing the soot to generate CO₂, and by using downstream CO₂ sensors to provide a more direct and more precise estimate of exhaust soot levels, in addition to their use in estimating exhaust CO₂ levels. By enabling an accurate, real-time estimate of exhaust soot levels, filter regeneration may be better determined and more accurately initiated. Additionally, the higher sensitivity of the CO₂ gas sensors may reduce the “dead-band” effect of resistive sensors and provide higher resolution between signals. This higher resolution may improve the ability to identify a degraded particulate filter. By improving accuracy in filter regeneration and filter diagnostics, the quality of exhaust emissions may be improved.

It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an engine and an associated particulate matter retaining system.

FIGS. 2-5 show high level flow charts illustrating routines that may be implemented for controlling and diagnosing filter regeneration and filter functionality, using CO₂ sensors, in the particulate matter retaining system of FIG. 1, according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for controlling a particulate matter retaining system, such as the diesel particulate matter (PM) retaining system of FIG. 1. As shown therein, the PM retaining system may include a diesel particulate filter, a filter substrate, and a CO₂ gas sensor positioned downstream of the filter and the substrate. As shown in FIG. 2, when the substrate is not heated, that is, the substrate is not enabled for oxidizing; the CO₂ sensor may be used to estimate exhaust CO₂ levels and adjust engine operation based on the estimated value. When the substrate is enabled for oxidizing (for example, when the substrate is electrically heated by a dedicated substrate heater, or non-electrically heated by hot exhaust gas), post-filter exhaust may be passed over the heated substrate and exhaust PMs, if present, may be oxidized to CO₂ using oxygen from the exhaust gas on the substrate upstream of the CO₂ sensor. Herein, the CO₂ sensor may be used to more accurately infer the presence of exhaust PMs, and an exhaust PM level, due to a direct correlation between the CO₂ gas generated on the substrate and the oxidized PMs. Accordingly, a CO₂ signature may be generated by the sensor during filter storing and/or regeneration conditions. As shown in FIGS. 3-5, by comparing an estimated CO₂ level (based on the sensor output) with an expected CO₂ level (based on, for example, engine operating or regeneration operation conditions), filter degradation may be identified, and distinguished from a degraded regeneration. In this way, a CO₂ sensor may be advantageously used to accurately identify the presence of PMs in post-filter exhaust, and provide more accurate control over filter functionality and regeneration efficacy. By reducing the need for resistive-sensing PM sensors, higher signal resolutions may be achieved, enabling improved emissions control.

FIG. 1 shows a schematic depiction of a vehicle system 6. The vehicle system 6 includes an engine system 8 coupled to a particulate matter (PM) retaining system 22. The engine system 8 may include an engine 10 having a plurality of cylinders 30. Engine 10 includes an engine intake 23 and an engine exhaust 25. Engine intake 23 includes a throttle 62 fluidly coupled to the engine intake manifold 44 via an intake passage 42. The engine exhaust 25 includes an exhaust manifold 48 eventually leading to an exhaust passage 35 that routes exhaust gas to the atmosphere. Throttle 62 may be located in intake passage 42 downstream of a boosting device, such as a turbocharger, (not shown) and upstream of an after-cooler (not shown). When included, the after-cooler may be configured to reduce the temperature of intake air compressed by the boosting device.

Engine exhaust 25 may include one or more emission control devices 70, which may be mounted in a close-coupled position in the exhaust. One or more emission control devices may include a three-way catalyst, lean NOx filter, SCR catalyst, etc. Engine exhaust 25 may also include PM retaining system 22 positioned upstream of emission control device 70. In one example, as depicted, PM retaining system 22 is a diesel particulate matter retaining system. Tailpipe exhaust gas that has been filtered of PMs, following passage through PM retaining system 22, may be further processed in emission control device 70 and expelled to the atmosphere via exhaust passage 35.

PM retaining system 22 may include one or more PM retaining devices, such as a diesel particulate filter (DPF) 102, to temporarily filter PMs from entering gases. DPF 102 may have a monolith structure made of, for example, cordierite or silicon carbide, with a plurality of channels inside for filtering particulate matter from diesel exhaust gas. PM retaining system 22 may further include substrate 104 positioned downstream of DPF 102, and a CO₂ gas sensor 106 positioned downstream of substrate 104. Substrate 104 may be heated non-electrically, for example by the passage of heated exhaust gas, or electrically, for example by the operation of a dedicated substrate heater 108, to oxidize post-filter exhaust PMs, or soot, into CO₂. A CO₂ signature of the post-filter oxidized PMs may then be obtained using the downstream CO₂ sensor.

Substrate 104 may have a monolith structure similar to DPF 102, made of, for example, cordierite, silicon carbide, etc. The substrate may not be catalyzed, consequently functioning similar to a filter. In one example, the diameter of substrate 104 may be configured to match the diameter of exhaust passage 35 so as to enable all exhaust gas to glow through the substrate. The length of substrate 104 may be adjusted based on desired storage properties. In the depicted embodiment, wherein substrate 104 may not have substantial soot storing functions, a shorter substrate may be used (for example, 50 mm or less). In alternate embodiments, wherein substrate 104 may be required to store soot in addition to oxidizing exhaust soot, a longer substrate may be employed.

Substrate 104 may be heated by substrate heater 108. In one example, as depicted, heater 108 may be an internal heater, such as an electrical resistance heater woven into the substrate. The electrical resistance heater may be controlled, using PWM control, to adjust the temperature of the substrate to a desired temperature. In alternate examples, the heater may be an external heater. In one example, the operation of substrate heater 108 may be based on exhaust gas temperature. Thus, the substrate heater may be turned on when the exhaust gas temperature is below a threshold (such as during filter storing), and turned off when the exhaust gas temperature is above the threshold (such as during filter regeneration). Alternatively, substrate heater operation may be based on filter conditions. Herein, the heater may be turned on when the filter is in a storing mode, and may be turned off when the filter is in a regenerating mode.

While the depicted example illustrates substrate 104 downstream of DPF 102, in alternate embodiments, substrate 104 may be included in DPF 102. During engine operation, substrate 104 may be heated, for example, due to the flow of heated exhaust gas over the substrate. Alternatively, substrate 104 may be heated by a dedicated heater. Heated substrate 104 may oxidize exhaust PMs on the substrate using oxygen from the exhaust. The oxidized PMs, such as soot, may thereby be combusted to generate CO₂. Downstream CO₂ sensor 106 may be configured to provide an accurate estimate of the generated CO₂ level. An exhaust PM level may then be determined or inferred by an engine controller 12 based on the estimated CO₂ level.

During engine operation, exhaust gas PMs may be retained and collected on DPF 102. Thus, an amount of post-filter exhaust PMs may be substantially low in the absence of filter degradation. Consequently, there may be substantially no difference between a CO₂ level sensed by the CO₂ sensor (that is, a sensor output) and a CO₂ level expected based on engine operating conditions. In the event of filter degradation (such as, due to a crack in the filter), soot may escape from the filter and may be detected due to a sudden increase in CO₂ levels estimated by the downstream sensor, due to oxidation of the released soot on the post-filter substrate. Post-filter exhaust PM levels may be determined based on CO₂ levels sensed by CO₂ sensor 106, downstream of substrate 104. For example, a real-time estimate of PM levels may be obtained. As further elaborated with reference to FIGS. 2-4, by monitoring the estimated CO₂ levels and/or inferred PM levels, DPF 102 operations may be assessed and filter diagnostic routines may be performed. In one example, in response to an increase in CO₂ levels above a threshold, during engine operation, filter saturation may be deduced and filter regeneration may be initiated. In another example, in response to CO₂ levels being above an upper threshold (or below a lower threshold) during and/or following filter regeneration, filter degradation may be diagnosed and filter diagnostic codes may be set.

Vehicle system 6 may further include control system 14. Control system 14 is shown receiving information from a plurality of sensors 16 (various examples of which are described herein) and sending control signals to a plurality of actuators 81 (various examples of which are described herein). As one example, sensors 16 may include exhaust gas sensor 126 (located in exhaust manifold 48), temperature sensor 128 and pressure sensor 129 (located downstream of emission control device 70), and CO₂ sensor 106 (located downstream of DPF 102 and substrate 104). Other sensors such as additional pressure, temperature, air/fuel ratio, and composition sensors may be coupled to various locations in the vehicle system 6. As another example, the actuators may include fuel injectors 66, throttle 62, DPF valves that control filter regeneration (not shown), etc. The control system 14 may include a controller 12. The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. Example control routines are described herein with reference to FIGS. 2-5.

FIG. 2 illustrates an example routine 200 for adjusting engine and filter operation based on feedback from a CO₂ sensor positioned downstream of a particulate filter in an engine exhaust. Specifically, by using the CO₂ sensor to estimate an exhaust CO₂ level and/or infer the presence of post-filter exhaust PMs, filter diagnostics may be performed during all engine running conditions including filter storing and filter regeneration.

At 202, the routine includes confirming that the engine is running. If the engine is not running, the routine may end. At 204, engine operating conditions may be estimated and/or inferred. These may include, for example, driver requested torque, engine temperature, exhaust gas temperature, exhaust gas composition, air-fuel ratio, etc. At 206, it may be determined whether a substrate positioned downstream of the particulate filter in the particulate matter retaining system (FIG. 1) is oxidizing. As such, the substrate may be enabled for oxidizing PMs when it is heated (for example, above a threshold temperature). In one example, the substrate may be electrically heated by operating the dedicated substrate heater. In another example, the substrate may be non-electrically heated by the flow of hot exhaust gas there-through. As such, when the substrate is hot and enabled for oxidizing, exhaust PMs, if present, may be collected and oxidized on the substrate using oxygen from the exhaust. Thus, if the substrate is not oxidizing, at 208, the CO₂ sensor may provide an estimate of exhaust CO₂ levels only, and an engine controller may be configured to adjust engine operation and perform engine component diagnostics based on the estimated exhaust CO₂ level. For example, the exhaust CO₂ level may be used to adjust an amount of EGR, an amount of boost, valve timing, etc. Similarly, the exhaust CO₂ level may be used to diagnose degradation in components such as EGR components, etc.

If the substrate is oxidizing, at 209, it may be determined whether filter regeneration has been enabled. If filter regeneration has not been enabled, then at 210, expected CO₂ levels (CO2_model_nonregen) may be estimated based on engine operating conditions. At 212, the estimated CO₂ level may be compared to a CO₂ signature output by the sensor (CO2_sensor_non-regen). At 214, filter diagnostics may be performed and engine operation may be adjusted based on the comparison between the estimated CO₂ level and the CO₂ signature of the sensor. Herein, the CO₂ sensor output may reflect a CO₂ signature corresponding to exhaust CO₂ levels including CO₂ generated by the oxidation of exhaust PMs on the heated substrate. Thus, the CO₂ signature may be used to infer the presence of post-filter exhaust PMs, and a post-filter exhaust PM level. In one example, inferring an exhaust PM level based on the estimated CO₂ signature may include determining what fraction of the CO₂ signature corresponds to non-soot exhaust CO₂ levels, and subtracting that fraction from the estimated CO₂ signature. Alternate algorithms may be used to compensate for exhaust non-soot CO₂ levels when determining the CO₂ level corresponding to the oxidized post-filter exhaust PMs. As further elaborated with reference to FIG. 3, adjusting engine operations and performing filter diagnostics based on a comparison between the estimated CO₂ level and the sensor output CO₂ signature may include diagnosing leak of exhaust PMs from a degraded filter, initiating filter regeneration and/or adjusting filter regeneration conditions (exhaust flow rate, exhaust temperature, etc.) based on the CO₂ signature.

Thus, during a first engine running condition, when the filter is storing and the substrate is not oxidizing, engine operation may be adjusted based on an exhaust CO₂ level estimated by the CO₂ sensor. Then, during a second engine running condition, different from the first engine running condition, when the filter is storing and the substrate is oxidizing (e.g., being heated by a substrate heater), engine operation may be adjusted based on the estimated CO₂ level of oxidized, post-filter exhaust PMs.

Now turning to FIG. 3, an example routine 300 is shown for diagnosing filter degradation and adjusting engine operation during engine running conditions where the substrate is oxidizing and the filter is storing. That is, the depicted routine may be performed during non-regeneration conditions. As such, during filter storing conditions, exhaust PMs may be retained in the filter and substantially no PMs may be present in the post-filter exhaust. Consequently, a degraded filter may be diagnosed due to the presence of PMs in the post-filter exhaust, based on a CO₂ signature determined by an exhaust CO₂ sensor downstream of the oxidizing substrate. By using a CO₂ sensor, a degraded filter may be distinguished from a marginal filter, and filter regeneration may be initiated accordingly.

At 302, the routine may include determining an expected CO₂ level (CO2_model_nonregen) based on the estimated engine operating conditions. For example, an expected CO₂ level may be based on an expected soot load, an exhaust air-fuel ratio, a duration of engine operation since engine start, an exhaust flow rate, etc. At 304, post-filter exhaust may be oxidized on the heated substrate. If present, exhaust PMs that have not been retained in the filter may be oxidized on the heated substrate using oxygen from the exhaust gas. In one example, during filter storing, when the exhaust gas temperature is below a threshold, the substrate may be electrically heated by operating a substrate heater. At 306, the CO₂ sensor's output may be determined (CO2_sensor_nonregen), and a CO₂ signature may be determined. The CO₂ signature may account for CO₂ from non-PM CO₂ sources, such as exhaust CO₂ from in-cylinder combustion events, as well as CO₂ generated due to the presence of PMs in the post-filter exhaust.

At 308, it may be determined whether the estimated CO₂ level as sensed by the CO₂ sensor (at 306) is higher than the expected CO₂ level (as determined based on models at 302). If yes, then at 310, it may be determined that soot (that is, exhaust PMs) are present in the exhaust. As previously elaborated, in the absence of filter degradation, exhaust PMs are retained in the filter. Thus, at 312, in response to the detection of post-filter exhaust PMs, filter degradation may be determined and indicated by setting a diagnostic code. Setting a diagnostic code may include, for example, illuminating a light on a vehicle dashboard to indicate filter degradation to the vehicle operator. Optionally, based on the CO₂ signature, a degree of filter degradation may also be indicated. For example, a smaller difference between the estimated level output by the sensor and the expected level may reflect a smaller degree of filter degradation (such as, release of PMs due to a small filter leak), while a larger difference may reflect a larger degree of filter degradation (such as, release of PMs due to a large filter crack). At 314, based on the indication of degradation, filter regeneration and engine operating conditions may be adjusted. In one example, in response to the indication of degradation, a subsequent filter regeneration operation may be performed for a shorter duration, at a relatively lower temperature, and/or in response to a lower threshold of stored PMs. In another example, in response to the indication of filter degradation, an engine load may be reduced to reduce the amount of PMs generated.

In comparison, if the estimated exhaust CO₂ level is below the expected level, then at 316, it may be determined that no soot is present in the exhaust. Consequently, at 318, it may be inferred that the filter is not degraded and can retain exhaust PMs. At 320, it may be further determined whether the estimated exhaust CO₂ level is above a threshold. If the estimated exhaust CO₂ level is contained below the threshold, then at 324, it may be determined that the filter has not yet reached its storage capacity and filter regeneration is not required. If the estimated exhaust CO₂ level is above the threshold, then at 322 it may be determined that the filter has approached its storage capacity and filter regeneration may be initiated to restore the filter's storage capacity.

Now turning to FIG. 4, an example routine 400 is described for diagnosing filter degradation during a filter regeneration operation. Additionally, the routine may enable the filter regeneration operation to be monitored, and allow a degraded filter regeneration operation to be distinguished from a degraded filter condition.

At 402, the substrate heater may be disabled (e.g., turned off) and filter regeneration may be initiated. As such, filter regeneration may be initiated based on engine operating conditions such as, engine torque, exhaust gas temperature, flow rate, and composition, etc. In one example, filter regeneration may be initiated responsive to the exhaust temperature being above a threshold, a threshold duration since the engine start having elapsed, a threshold duration since a previous filter regeneration operation having elapsed, etc.

As such, during filter regeneration, hot exhaust gas may be directed at a predetermined flow rate through the filter to burn particulate matter stored therein during a previous storing cycle. Thus, during regeneration, the substrate heater may not be operated while the heated exhaust is advantageously used to also heat the downstream substrate. In one example, the heater may be controlled responsive to exhaust temperature. For example, when the exhaust gas temperature is above a threshold (such as above a regeneration temperature), the substrate heater may be turned off, and when the exhaust gas temperature is below the threshold, the substrate heater may be turned on.

At 404, the regeneration conditions may be estimated. These may include estimating an exhaust gas flow rate and temperature to be used for the regeneration. Additionally, the soot load on the filter before regeneration may be estimated. At 406, based on the estimated regeneration conditions, a corresponding expected CO₂ regeneration signature/CO₂ level may be determined (CO2_model_regen). In one example, the expected CO₂ signature may be a CO₂ level profile expected for the duration of the regeneration. As such, the CO₂ level may increase or decrease depending on conditions during filter regeneration. In one example, the CO₂ level may be expected to increase soon after the regeneration is initiated as the stored PMs are burned off, and then the CO₂ level may decrease. At 408, post-filter exhaust PMs may be oxidized on the heated substrate using oxygen from the exhaust gas. At 410, a CO₂ level of the post-filter exhaust (CO2_sensor_regen) may be estimated by the downstream CO₂ sensor. Accordingly, an estimated CO₂ signature may be determined. The estimated CO₂ level may reflect CO₂ from non-PM sources, such as CO₂ from in-cylinder combustion events, as well as CO₂ released from the burning of PMs stored in the filter. Additionally, the estimated CO₂ level may reflect CO₂ from post-filter exhaust PMs, if present, that are oxidized on the heated substrate. At 412, the sensor output estimated regeneration CO₂ level may be compared to the model based expected regeneration CO₂ level and it may be determined if the sensor output is higher than the modeled value. If yes, then at 414, it may be determined that soot is present in the exhaust. As previously elaborated, in the absence of filter degradation, exhaust PMs are retained in the filter. Thus, at 416, in response to the detection of post-filter exhaust PMs, filter degradation may be determined and indicated by setting a diagnostic code. Optionally, as previously elaborated in FIG. 3 (at 312), based on the CO₂ signature, a degree of filter degradation may also be indicated. At 418, based on the indication of degradation, engine operating conditions and/or regeneration conditions for the same and/or a subsequent regeneration operation, may be adjusted. In one example, in response to the indication of degradation, the filter regeneration operation may be stopped (for example, immediately) to reduce the risk of degraded exhaust emissions. In another example, a regeneration exhaust flow rate may be decreased, a regeneration exhaust temperature may be lowered, a duration of regeneration may be decreased, and/or regeneration may be initiated at a higher soot load. In another example, in response to the indication of filter degradation, the engine load may be reduced to reduce the amount of PMs generated.

If the sensor output is not higher than the expected model value, then at 420, it may be determined that no soot is present in the exhaust, and it may be inferred that the filter is not degraded at 422. At 424, it may be determined if the estimated regeneration CO₂ level (or profile) is lower than a threshold. As such, during filter regeneration, an increase in CO₂ levels (such as, a sudden burst in CO₂ level following the initiation of regeneration) may be expected due to the burning off the stored soot. Thus, at 426, filter regeneration degradation may be determined when the estimated regeneration CO₂ level is lower than the threshold level. That is, it may be determined that the filter regeneration operation is degraded and that the soot stored on the filter has not been completely removed. Optionally, an engine controller may further adjust regeneration conditions, for the same and/or a subsequent regeneration operation, responsive to the indication of filter regeneration degradation at 418. For example, in response to the filter not being degraded but the filter regeneration operation being degraded, the exhaust flow rate for a subsequent regeneration may be increased, the exhaust temperature used for regeneration may be raised, the duration of regeneration may be increased, and/or regeneration may be initiated at a lower soot load.

In comparison, if the estimated regeneration CO₂ level is below the threshold, then at 428, it may be determined that the filter is not degraded and that the filter regeneration operation is also not degraded.

Thus, during filter storing, an engine controller may electrically heat the substrate with the heater, estimate a post-filter exhaust CO₂ level to infer the presence of post filter exhaust PMs with the CO₂ sensor, and control filter regeneration and adjust engine operation based on the estimated exhaust CO₂ level. Similarly, during a filter regeneration condition, different from the filter storing condition, the controller may disable the heater and non-electrically heat the substrate using the hot exhaust gas used for filter regeneration. The controller may then estimate a post-filter exhaust CO₂ level to infer the presence of post filter exhaust PMs with the CO₂ sensor, and control filter regeneration and adjust engine operation based on the estimated exhaust CO₂ level. In either condition, the sensor may also enable the controller to determine filter degradation based on a CO₂ signature generated from oxidized post-filter PMs.

While the depicted routines illustrate adjusting engine operation and determining filter degradation based on the CO₂ signature, it will be appreciated that the engine controller may be further configured to diagnose CO₂ sensor degradation based on the estimated CO₂ level. Diagnosing CO₂ sensor degradation may include setting a corresponding diagnostic code.

In one example, during filter storing, CO₂ sensor degradation may be diagnosed by estimating a first CO₂ level with the substrate heater on, estimating a second CO₂ level with the substrate heater off, and indicating CO₂ sensor degradation when a difference between the first CO₂ level and the second CO₂ level is less than a threshold. For example, when it is known that the filter is not degraded, by comparing the CO₂ level estimated by the CO₂ sensor with the heater on and off, sensor degradation may be identified due to the sensor not detecting the presence of an anticipated amount of CO₂.

In another example, during filter regeneration, CO₂ sensor degradation may be diagnosed by determining a first rate of change in the estimated CO₂ level over at least a duration of the filter regeneration operation, determining a second rate of change in the pressure across the filter over the same duration, and indicating sensor degradation when a difference between the first and second rate of change is greater than a threshold. Alternatively, sensor degradation may be indicated when a ratio between the first and second rate of change is greater than a threshold. Thus, for example, when it is known that the filter is not degraded and the filter is regenerating, by comparing a parameter reflective of filter regeneration (such as a change in pressure, or temperature, or mass, etc., across the filter) over a duration of the operation with the change in the output from the sensor over the same duration, sensor degradation may be identified due to the sensor output not changing at a rate proportional (or normalized) to the rate of change of the regeneration parameter across the filter.

Now turning to FIG. 5, an alternate example routine 500 is depicted for identifying particulate filter degradation based on the output of a downstream CO₂ sensor. At 502, it may be confirmed that the engine is running. As such, if the engine is not running, the routine may end. At 504, engine operating conditions may be estimated, measured and/or inferred. At 506, CO₂ levels may be estimated and/or inferred based on the estimated engine operating conditions (CO2_model). In one example, a CO₂ model may be used to estimate an expected CO₂ level based on the engine operating conditions. The level estimated by the CO₂ model may take into account CO₂ levels anticipated from exhaust PMs, exhaust CO₂, unburned and partially burned exhaust hydrocarbons, etc. At 508, the modeled CO₂ level may be compared to the output of the CO₂ sensor. At 510, it may be determined whether the CO₂ sensor output is higher than the modeled CO₂ level. If no, then at 514, it may be assumed that the particulate filter is functioning properly. In comparison, if the CO₂ sensor output is higher than the modeled CO₂ level, then at 512, it may be assumed that the particulate filter is degraded and a filter degradation indicator may be illuminated. The degradation indicator may be, for example, a light (such as a malfunction indicator light) on the vehicle dashboard, or a message in a message center presented to the driver.

It will be appreciated that while one or more of the routines of FIGS. 2-5 may be illustrated with reference to diesel particulate filters, this is not meant in a limiting sense, and the same routines may be similarly applied towards the diagnosis of degradation of other particulate filters, such as a gasoline particulate filter in a gasoline engine exhaust system.

In this way, an exhaust CO₂ sensor may be used to determine exhaust CO₂ levels as well as to accurately infer an exhaust PM level. Additionally, based on the estimated CO₂ level during filter regenerating and non-regenerating conditions, filter diagnostics may be performed. Based on differences between the estimated level and an expected level, engine operation and/or filter regeneration operation may be further adjusted. By using higher resolution CO₂ sensors, filter diagnostics may be performed with higher precision.

Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system.

It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure. 

The invention claimed is:
 1. A method for monitoring particulate matter (PM) emissions from an engine exhaust including a particulate filter, comprising, exhausting PMs from the particulate filter to a downstream substrate; controlling temperature of the substrate via a heater, where exhausted PMs are oxidized on the substrate upstream of a CO₂sensor; and indicating filter degradation based on a CO₂ signature of the oxidized, post-filter exhaust PMs indicated by the CO₂sensor.
 2. The method of claim 1, wherein the CO₂ signature is based on a CO₂ level estimated by the CO₂ sensor positioned downstream of the substrate, the method further comprising determining an amount of exhausted PMs from the particulate filter based on the CO₂ level.
 3. The method of claim 2, wherein oxidizing exhaust PMs on the substrate includes operating the substrate heater to heat the substrate, and oxidizing exhaust PMs on the heated substrate using oxygen from exhaust gas downstream of the particulate filter.
 4. The method of claim 2, wherein the CO₂ signature includes an estimated CO₂ level that is generated during a filter storing operation.
 5. The method of claim 4, further comprising, initiating filter regeneration in response to the CO₂ level being higher than a threshold during filter storing.
 6. The method of claim 4, wherein the indication includes, indicating filter degradation when the CO₂ level is greater than a threshold.
 7. The method of claim 4, wherein the indication includes, indicating degradation when the estimated CO₂ level is greater than an expected CO₂ level, the expected CO₂ level determined based on engine operating conditions.
 8. A method of operating an engine exhaust system including a particulate filter and a downstream electrically heated substrate, comprising, during substrate non-heating, adjusting engine operation based on an exhaust CO₂ level estimated by a sensor; and during substrate heating, oxidizing post-filter particulate matters (PMs) on the substrate and adjusting engine operation based on a CO₂ signature from the sensor indicative of PM levels downstream of the particulate filter.
 9. The method of claim 8, wherein the engine is a gasoline engine, and wherein the adjusting engine operation based on the exhaust CO₂ level estimated by the sensor includes adjusting engine EGR operation, and wherein adjusting engine operation based on the CO₂ signature includes initiating particulate filter regeneration.
 10. The method of claim 9, wherein the sensor is a CO2 sensor, and the substrate is upstream of the CO₂ sensor, and wherein during the non-heating condition, the filter is storing and the substrate is not oxidizing and during the heating condition, the filter is storing and the substrate is oxidizing.
 11. The method of claim 10, wherein the exhaust PMs are oxidized on the heated substrate using oxygen from exhaust gas.
 12. The method of claim 9, wherein adjusting engine operation based on the CO₂ signature includes initiating filter regeneration when the estimated CO₂ level of oxidized, post-filter exhaust PMs is greater than a threshold.
 13. The method of claim 12, further comprising, indicating filter degradation based on the CO₂ signature indicating a level of oxidized, post-filter exhaust PMs.
 14. The method of claim 13, wherein the indication includes determining an expected CO₂ level based on engine operating conditions, comparing the expected CO₂ level to the estimated CO₂ level, and indicating filter degradation when the estimated CO₂ level is greater than the expected CO₂ level, wherein indicating filter degradation includes setting a diagnostic code.
 15. An engine exhaust system, comprising: a particulate filter; a CO₂ sensor coupled downstream of the filter; a substrate positioned upstream of the CO₂ sensor and downstream of the particulate filter; and a controller having instructions thereon to, during filter storing, heat the substrate; estimate a CO₂ level corresponding to exhaust PMs oxidized on the heated substrate; initiate filter regeneration based on the estimated CO₂ level; and determine filter degradation based on the estimated CO₂ level.
 16. The system of claim 15, wherein heating the substrate includes, turning on a substrate heater when an exhaust gas temperature is below a threshold to electrically heat the substrate; and turning off the substrate heater when the exhaust gas temperature is above the threshold to non-electrically heat the substrate using heated exhaust gas. 